You are correct that it is untranslatable into classical Latin. Consequently, I have done what the Romans did whenever the resources of their own language failed, viz., I have introduced a Greek word and paraphrased. 'Taenia' means a 'fillet' or 'headband' in classical Greek, but 'film' in the modern language. 'Festa', in the sense which you want, is not found in prose before Pliny Sr.; it is neuter plural, not feminine singular. (The singular is not found in prose until Lactantius). 'Taeniarum' ('of films') is genitive of description. Again, a prepositional phrase used adjectivally--here 'inter (omnes) gentes'--is rare, but not unprecedented, in classical prose. I would eliminate 'omnes' unless films from all or at least most nations are being submitted. There is nothing for it but to coin an adjective 'Miamianus' from 'Miami', treating the latter as if it were a second declension plural. Even if I knew the etymological meaning of 'Miami', it would not be recognisable if rendered into Latin.

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Note added at 2 hrs (2009-01-11 21:02:49 GMT)
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Thank you, although your praise is a bit excessive. I would omit 'omnes'. You may capitalise it: in fact, the Romans themselves used only capitals.